3D images are images, which contain 3D (3-dimensional) information about objects on the image. Observer therefore can see these objects as real ones - spatially - in 3D. The objects aren't flat - and aren't on one surface - but are spatial and placed in the space (they can even go out of the image - for example out of a PC monitor).
Source + more information: 3DJournal Club - 3D images + 3D glasses